The present invention relates to radiation treatment of cancerous tissue internally within the human body. More specifically, the present invention pertains to a high activity/high dose iridium source adapted for remote afterloader implantation in living tissue.
In general the irradiation of tissue as a means to reduce or eliminate malignancy has been known for many years. The specific procedure and equipment required for any given treatment, however, varies according to numerous factors including the location of the cancer within the body, its level of development and spread, and the age and condition of the patient.
For example, United States patents to Wappler, U.S. Pat. Nos. 2,322,902 and 2,439,438 and Packer, U.S. Pat. No. 3,458,365, relate to the manufacture of radium and xenon gas "seeds" which may be used to treat cancer by physically implanting the seeds at the cancer site in the patient's body. As described in the Packer patent, these implants may be of low dosage, thereby facilitating the permanent implantation thereof as well as permitting safe handling by doctors and other personnel during surgical installation. This form of irradiation treatment is limited to those regions of the body in which placement of the seeds can be effected without undue trauma to adjacent normal tissue.
Another known cancer treatment procedure involves the insertion of a radioactive source through a guide tube, which may be an elongate needle or catheter. The present invention relates generally to this radiation source placement technique.
While the use of a guide tube (needle or catheter) offers relatively safe and easy access to many parts of the body, it is not without its limitations. First, the placement of the guide tube, while less invasive than alternative direct surgical placement, nevertheless can traumatize tissue along its path of insertion. This is particularly true for treatment of, for example, brain tumors, where a hole to receive the guide tube must be drilled through the cranium and brain tissue. Any drilling of brain tissue causes irreparable damage thereto and consequently such drilling is desirably kept to a minimum.
Second, for low intensity radiation treatments, the source of radiation must remain resident for extended periods of time, often in excess of several days. As the patient will not normally be hospitalized during this entire extended treatment cycle, the guide tubes must be inserted, positioned, and terminated such that the patient may undertake much of his normal daily routine.
Known indirect guide tube products typically employ an iridium/platinum alloy of approximately 30% iridium. These products are often used in connection with low intensity, source-resident type procedures, that is, where the source remains in the body for extended durations. Known iridium alloy sources used for such irradiation generally exhibit activity levels between about 0.5 and 25 millicurie/centimeter. (The overall activity level thus varies as a function of the length of the active region, which may be several centimeters.)
Alternatively, iridium/platinum alloy sources of proportionately larger diameter have been fabricated to achieve the higher radiation activity levels necessary for short term treatment procedures. As noted, placement of these larger diameter sources results in correspondingly increased damage to normal tissue.
The smallest source assemblies heretofore used have been about 1 millimeter in diameter.
Furthermore, known guide tube sources are fabricated by welding or otherwise affixing an iridium alloy source to the end of a steel "fish" or delivery wire. Insertion of these relatively large sources can be frustrated where the guide tube has been oriented, for medical reasons, along a curved contour. In fact, stresses induced during source insertion have been known to break the source from the delivery wire at its point of welded attachment. This breakage problem is particularly acute where it is expected that the iridium source will be reused for a number of surgical procedures. Repeated flexure ultimately causes joint fatigue and failure.
Radioactive iridium is a common source of irradiation for cancer treatment because it has a convenient half life and emits gamma rays of suitable energy. The useful isotope is iridium 192, which has a half life of about 74 days. This is sufficiently long to permit use of the source at some time and distance from its creation. It emits gamma rays of a number of useful energies in the range of hundreds of KeV and less than about 0.5 MeV. One such gamma ray energy is about 484 KeV. This is energetic enough to pass out of the guide tube and through adjacent tissue to irradiate the tumor but is not so energetic as to reach more remote parts of the body to the detriment thereof. That is, the radiation can be relatively concentrated at the tumor without destroying too much healthy tissue.
A problem with certain prior art iridium guide tube sources has been attributed to their fabrication. Iridium 192 is produced by the irradiation of iridium 191 in a nuclear reactor. At least partly because of their size and shape, the sources were assembled with respective delivery wires before irradiation, which resulted in the irradiation of these wires, rendering them radioactive as well, with undesirable half lives and energies.